1. Field of the Invention
The present invention relates to a process for making conducting connections in dielectric substrates by vacuum deposition of metals on the surface of and inside the through holes and the product obtained thereby, especially for application in hybrid integrated thin film circuits.
2. Description of the Prior Art
To make low-resistance electrical connections between the two sides of a dielectric substrate of a hybrid microelectronic thin film circuit it is known to use as connection paths holes made at certain points of the substrate and to coat the vertical walls of the holes with layers of metal connected to the circuit elements on the two sides of the substrate.
In this manner connection is assured in an integrated and economical manner. The length of the connection between the two sides is reduced to the minimum so that the components welded on one side of the substrate, hereinafter called the front, are connected to the other side, hereinafter called the back, by the shortest possible path.
The technique of conductive coating of the side walls of the holes is certainly simpler and more economical to perform industrially than the conventional technique which calls for making each individual connection with wires or metal strips inserted in the holes and welded to the two sides of the substrate.
The main problem is to establish how and in what phase of the hybrid thin film circuit production process to perform the operations creating the metallized holes.
The most obvious and widely used known method is to drill the substrate at the beginning of processing, i.e. when no metal deposits of any kind are present on the substrate. In this manner the metal layers deposited subsequently on both sides of the substrate also cover the walls of the holes already made.
The main drawbacks of this known method are the following. Once the substrates are drilled they can be used only for a specific type of circuit, i.e. requiring holes in those positions. Some equipment used in the fabrication of the hybrid thin film circuits uses the technique of suction and creation of vacuum to hold the substrates fixed to the supports, especially in the initial more delicate phases of pattern transfer and subsequent selective galvanic growth of the conductive paths on the substrate. The technique is not usable if the substrates are studded with holes. In photolithographic operations it is mandatory to use the holes as datum points for aligning masks with the substrate, giving rise to problems related to the mechanical precision with which the holes are located and the much higher optical precision of photolithography.
One way of avoiding the aforementioned drawbacks is, for example, the one set forth in Italian patent application filed 19 Mar. 1986 under number 19803-A/86 in which is claimed a process for making metallized holes in a substrate with conductive and resistive paths characterized in that it comprises in succession the following phases: drilling of a flat substrate with conductive and resistive paths; repeated washing, first coarse and then fine, of the drilled substrate to remove drilling debris and preparation of the circuit for the following treatment; drying of the circuit; deposition of a substance capable of aiding adhesion over the entire surface of the two faces of the metallized substrate; another drying of the circuit; chemical deposition of a thin layer of copper over the entire surface of the two faces of the substrate; another drying of the circuit; masking of the two faces of the substrate excepting the holes and the immediately adjacent areas; galvanic growth of copper in the areas not masked; removal of masking; chemical removal of the copper from the previously masked areas of the substrate.
The main drawback of this process is that it requires numerous processing steps of chemical and electrogalvanic deposition to metallize the walls of the holes. Furthermore adherence of the metallization inside the hole is not optimal.
Another known process is the one described in the article by R.J. Thompson, B.W. Whitaker "Conductive via processing utilizing RF sputtering and selective electro deposition for hybrid thin film circuits", Proceedings 1982 of the 32nd Electronic Components Conference, San Diego, Calif. According to the aforementioned process, the metal layers are deposited only on the front of the substrate and drilling is performed when the operations necessary for obtaining the thin film hybrid circuit have been completed only for this side. Conductive connection through the holes is secured during deposition of the metal layers on the back of the substrate. However, thicker layers than those normally needed must be deposited.
Furthermore the hole walls must be perfectly vertical and lastly special procedures must be used in the deposition system to assure that the metals deposited from the back reach during deposition the metallizations of the front which face the edge of the hole. It is inconceivable to deposit metal layers on the front also because the existing circuit would have to be protected in some way.
Therefore, the object of the present invention is to overcome the aforementioned drawbacks, especially those inherent in the process described in the aforementioned article, and provide a process for making conducting connections in dielectric substrates by the vacuum deposition of metals on the surface and inside the through holes, the principal feature of which is the creation of conductive rivets protruding beyond the edge of the holes on the front of the substrate.